The methods for load testing in present use comprise either loading the head of the pile with an appropriate weight or applying a downward force thereon by means of hydraulic jacks acting against a ballast or against a reaction member held down by anchors sunk into the soil at a distance from the pile or foundation to be tested.
An apparatus in use with the first method consists of a box or platform which is supported on top of the pile to be tested and is loaded with earth, sand, pig iron or the like up to the desired test load. This method, however, is very crude and does not lend itself to exact measurements. It has been improved by a similar arrangement, wherein a box or platform is placed with its center above the test pile, while both of its ends are supported on cribbing or the like. A calibrated jack is placed on top of the pile and, while actuated, presses against the bottom of the box or platform which is duly reinforced by beams or girders. The latter is either lifted off the cribbing if the test load equals the load put onto it, or its ends remain on their supports if it is intentionally loaded by a greater weight than the test load to be applied, the latter method being preferred in most cases. This method enables exact measurement and lends itself to fully loading the pile within a relatively short time period.
Both methods, however, suffer from the drawback that the soil is preloaded and compacted in the areas supporting the platform or box prior to and during the test, which falsifies the test results, giving in fact higher values of soil resistance and bearing capacity.
Some of the above drawbacks are obviated by a device in use with the second method, wherein two or more piles to be used as anchors are driven into the soil as far from the test pile as possible. A girder or beam of sufficient strength and resistance against bending is attached to the upper ends of the anchors with its center passing above the test pile, and a calibrated hydraulic jack is positioned between the top of the test pile and the underside of the girder or beam. For applying the desired load onto the test pile the jack is actuated against the beam which transfers the force to the anchors, thus pressing the test pile into the soil.
This method requires a very heavy beam or girder whenever the anchors are positioned sufficiently far from the test pile so as not to influence the distribution of forces and stresses in the soil. Consequently a heavy crane is required adding to the expenses of the loading test. With a view of reducing these expenses, soil anchors are often sunk at a shorter distance from the test pile, which allows for a lighter beam or girder, but may give completely erroneous results, owing to the upwardly directed forces acting on the anchors which are transferred to the test pile by the surrounding soil.
A further shortcoming of the methods and apparatus enumerated above is that for measuring the consolidation settlement behavior over a larger time period the loading force requires regular and frequent adjustment by means of the jack or jacks, since the beams or girders used with these apparatus are stiff and rigid, so that the load is soon reduced when the test pile settles.
The main objects of the present invention are to overcome the above-mentioned shortcomings and to provide a test-loading rig that (1) allows the positioning of the anchors at a sufficiently large distance from the test pile, (2) is of light weight lending itself to easy handling and erection, and (3) enables the measuring of consolidation settlement over a long period of time without the necessity of frequently adjusting the load on the test pile by operating a jack or jacks.